Earth boring bits such as rock bits for the oil field and mining industries have long been provided by rotary drill bits having milled steel teeth formed on rotary cone cutters in circumferentially extending rows. The height of the milled teeth are measured from a base to a crest. The softer the rock, the larger the height of the teeth which may be used, providing for more aggressive rock bits capable of drilling more quickly through the softer earth formations. In harder formations less aggressive bits are used having teeth of smaller height. Cutting surfaces for rock bits have also been provided by embedding inserts into rotary cutters and bit bodies. Tungsten carbide inserts (“TCI”) and polycrystalline diamond compacts (“PDC”) have been used. The tungsten carbide inserts typically have a tungsten carbide insert body with one end formed to provide a cutting surface. The PDC inserts typically have an insert body formed of tungsten carbide with one end having a polycrystalline diamond cutting surface.
For rotary cone rock bits, the tungsten carbide insert bodies typically extend outward of the rotary cutters in a cantilevered arrangement. Compressive and torsional loads on the cantilevered insert bodies often lead to failure. Erosion of steel material of the rotary cutters from around the insert bodies is also a common failure mode. Increasing the amount of steel material of the rotary cutters around the insert bodies can provide more material for erosion and reduce the cantilevered length the insert bodies extend, reducing the compressive and torsional loads to provide longer service life for the bit. Although service life is increased, increasing the amount of cutter material around the insert bodies reduces an effective tooth height for the inserts which provides a less aggressive bit, reducing the speed at which the bit will move through earthen formations.
One example of an earth boring bit is the rotary cone rock bit. Rotary cone rock bits have a bit body with an upper end adapted for connection to a drill string and typically three bit legs which extend downward from the body to provide support arms. A bearing shaft extends inward and downward from each bit leg. A conventional rock bit bearing shaft is cylindrical and rotatably receives a rotary cutter provided by a cutter cone. The cutter cone is generally mounted on each bearing shaft and supported rotatably on bearings acting between the spindle and the inside of a spindle-receiving cavity in each cutter cone. The cutter cones have teeth, inserts or compacts on their exteriors for disintegrating earth formations as the cones rotate on the bearing shafts. One or more fluid nozzles are often formed on the underside of the bit body. The nozzles are typically positioned to direct drilling fluid passing downwardly from the drill string toward the bottom of the borehole being drilled. Drilling fluid washes away material removed from the bottom of the borehole and cleanses the cutter cones, carrying the cuttings and other debris radially outward and then upward within an annulus defined between the drill bit and the wall of the borehole.